The basic concept of the magnetic properties of grain-oriented electrical steel sheets was studied for the first time when the magnetic anisotropy of a single crystal of iron was discovered in 1926 [K. Honda and S. Kaya: Sci. Reps., Tohoku Imp. Univ., 15 (1926), p. 721]. It has become possible to produce grain-oriented electrical steel strips having greatly improved magnetic properties since a process for producing a material having a {110} &lt;001&gt; texture was invented by N. P. Goss (U.S. Pat. No. 1,965,559).
The aggregation of the grains having a {110} &lt;001&gt; orientation in electrical steel strips is achieved by utilizing a catastrophic phenomenon of grain growth called secondary recrystallization. The control of secondary recrystallization essentially requires the control of a primary recrystallization texture and structure prior to the secondary recrystallization thereof and the control of an inhibitor, i.e. a fine precipitate, or an element of the intergranular segregation type. The inhibitor inhibits the growth of any grains Other than those having a {110} &lt;001&gt; orientation in the primary recrystallization texture and enables the selective growth of the grains having a {110} &lt;001&gt; orientation.
The following are the three typical processes which are known for the industrial manufacture of grain-oriented electrical steel strips or sheets:
(1) The process as disclosed by M. F. Littmann in U.S. Pat. No. 2,599,340 (Japanese Patent Publication No. 3651/1955) which employs two steps of cold rolling utilizing MnS as the inhibitor; PA1 (2) The process as disclosed by Taguchi and Sakakura in U.S. Pat. No. 3,287,183 (Japanese Patent Publication No. 15644/1965) which adopts a reduction rate exceeding 80% in final cold rolling utilizing an inhibitor comprising AlN and MnS; and PA1 (3) The process as disclosed by Imanaka et al. in U.S. Pat. No. 3,932,234 (Japanese Patent Publication No. 13469/1976) which employs two steps of cold rolling utilizing an inhibitor comprising MnS (or MnSe) and Sb. PA1 "Although the movement of the magnetic domain walls plays a principal role in the process of static or low frequency magnetization, it is considered better in a high frequency range to achieve magnetization by domain rotation, since in a high frequency range, the domain walls are not only difficult to move, but also the movement thereof produces a loss of energy"
These processes have made it possible to produce on a commercial basis grain-oriented electrical steel Strips in which the grains having a {110} &lt;001&gt; orientation have so high a degree of sharpness that the strips have a magnetic flux density (B.sub.8 value) of about 1.92 tesla. With a reduction of sheet thickness, however, the inhibitor exhibits a sensitive behavior of change through the interface which makes it difficult to produce thin grain-oriented electrical steel strips on an industrial basis. The main strips which are industrially available have, therefore, a thickness which is not smaller than 0.20 mm.
The core loss of grain-oriented electrical steel strips in a high frequency range increases in proportion to the square of their thickness, as reported by, for example, R. H. Pry and C. P. Bean in J. Appl. Phys., 29 (1958), p. 532. Therefore, it is essential to make a strip having a small thickness if it is desirable to obtain a sheet having a low core loss.
In 1949, M. F. Littmann disclosed a process for producing very thin silicon steel strip in U.S. Pat. No. 2,473,156. This process comprises cold rolling a starting material having a {110} &lt;001&gt; crystal orientation and subjecting it to a recrystallizing treatment, and does not use any inhibitor. The products of the process had a thickness of 1 to 5 mils (25.4 to 127 microns), a magnetic flux density (B.sub.8 value) of 1.600 to 1.815 teslas, and a core loss of 0.26 to 0.53 W/lb. (0.44 to 0.90 W/kg) at a frequency of 60 Hz and a maximum magnetic flux density of 1.0 T. This process is still used for producing very thin electrical steel strips.